"If you want to write a novel that readers will remember decades or even
centuries later, learn from the masters and populate it with one or more
extreme characters. You’ll find they’ll not only linger in a reader’s
mind, but they’ll give your story energy and heighten your own interest
in writing it.

Sounds great! But how do we create extreme characters?

1. Your protagonist should have a "deformity, affliction or peculiarity" that is the "driving force in your story".

For example,

"In The Phantom of the Opera, the phantom’s disfigurement
dominates the story. His fear that he will frighten off people,
especially the woman he loves, causes him to hide in the bowels of the
Paris Opera House and wear a mask."

2. Use your characters flaw(s) to reveal the kind of person they are

Marjorie Reynolds writes:

"A hero is not a perfect person who conquers all. He makes mistakes. He
usually possesses a tragic flaw (hubris or stubbornness, for example)
that makes him vulnerable to his enemies. A hero is someone with all the
faults of an ordinary person but with the strength of character to
struggle to the point of death. He won’t give up.

....

"To win at the end, he must struggle and push himself beyond what he
believes he can do. He must go beyond the point where we would stop.
You don’t have to tell us he’s a hero. We can see he is."

Characters must have faults and they must struggle to overcome them, even if this--especially at first--doesn't seem possible.

3. Show your characters strengths and weaknesses through action. Don't describe them to the reader.

5. Make your character admirable.

Someone once said--I remembered it because it made a lot of sense to me--that your character doesn't have to be nice but they do have to pursue justice.

I think that's what makes anti-hero's possible. An anti-hero isn't a good person; sure, maybe they are underneath it all, but if so we're talking about way underneath. What makes an anti-hero the good guy/gal is his/her cause, his/her goal. He/she is writing a wrong, fighting the good fight.

Five ways to gauge whether your character is extreme:

a. Does your character do something--fight vampires, find relics, break out of prison after digging a tunnel with a spoon--that no ordinary person could or would?

b. Does your character have extreme/strong emotions to go along with their extreme behavior?

In order for readers to believe someone would spend years digging a hole with a spoon (Shawshank Redemption), they need a context. They need to understand the character's emotional motivation through that character's actions.

c. Does your character have an extreme goal?

In Lord of the Rings, Frodo--a hobbit who knows nothing of the larger world, or fighting, or the evil machinations of wizards, and certainly nothing of the evil of the ring--ventures out to do what no one else has: destroy the One Ring.

Let me give you a real life example: Jerry Gretzinger and his map. 50 years ago Jerry Gretzinger created a map. The map, and his idea of it, evolved over time and--due to the nature of the map-- there is no clearly defined end state. In Jerry's words:

“The map began as just a doodle. I just made little rectangles and crosshatched them. Carefully. And I just kept adding rectangles and I put a river in....and some railroad stations. But there was this moment when I came to the edge of that sheet of paper…Got out another sheet of paper and put the two together…and I think I taped them together. That’s when I realized that it kind of had a life of its own.”(Mapping “The Void”)

However, Jerry says his goal is to work on his map till he dies. That's his goal. When the end comes, he wants to expire in his studio, mixing his paints. To work on some one thing for 50 years and to want to keep on for another 20 or 30 ... wow. That's extreme. Also, it's compelling. After I read about Jerry's map I couldn't get it out of my head.

(Perhaps the example of Jerry and his map should be under (a), above, but I think it fits here as well.)

d. Is your character's backstory extreme?

Extreme emotions, extreme goals, extreme behavior all needs a good explanation. Something outstanding, something unusual must have happened to account for, to explain, why your protagonist is how he/she is.

Superheroes often have interesting and extreme backstories to explain the character's behavior. Superman for example.

e. Does your character, at any point, stand alone?

Extraordinary characters will go where ordinary characters won't. Think of Mitch McDeere in The Firm. He has a lot of help from his friends and family, but in the end he stands alone.